The present invention relates generally to a chemically activated high capacity, microfiltration, composite polymer and silica-based membrane sorbent.
Various sorbents/ion exchange materials are available for metal/nitrate ion sequestration. Unfortunately, however, all of these suffer from the disadvantage that they possess at most two or three functional groups capable of ion interaction per attachment site. Additionally, these conventional materials are in bead (porous) form and thus, are not suited for effective utilization in convective flow applications.
As a specific example of this, ion-exchange resins (IERs), such as strong acid or weak acid cationic exchangers, have been used extensively to recover heavy metals and/or to prepare high quality water. The typical theoretical capacity of these IERs is five meq/gram (see xe2x80x9cIon-Exchange Resins and Related Polymeric Adsorbentsxe2x80x9d, Technical Bulletin AL-142, Aldrich Chemical Company). This capacity is quite low. For example, if one considers a typical charged metal ion such as nickel (II) a maximum uptake of only 0.15 gram of metal per gram of IER is possible. Further, the requirement for the regeneration of these IERs is a serious disadvantage as it produces concentrated waste solutions. Still further, the use of ion exchange beads requires column operations with high pressure drops and the rate of metal ion uptake is thereby limited by diffusion control.
Of course, there are many industrial situations where it is required to convert metal ions from the solution state to a solid form. This is done in order to facilitate the disposal of such metal species. In still other situations subsequent regeneration is not a consideration and/or a liquid volume reduction and entrapment of low levels of radioactive ions in a solid form is required. In these instances and applications, IERs have a significant cost disadvantage.
It is known, however, that liquid volume reduction and metal ion entrapment may be achieved using inexpensive, commercially available, high molecular weight cut-off ultrafiltration or microfiltration membranes in which internal surface areas range from 50-200 m2/gm. The most inexpensive materials used to prepare such membranes are cellulose and its derivatives, cellulose acetate and cellulose triacetate. Examples of such membranes are disclosed, for example, in U.S. Pat. Nos. 4,824,870 and 4,961,852 both to Pemawansa et al.
Both flat sheet and wide bore hollow fiber (200-300 xcexcm in diameter) configurations are readily available commercially. However, direct use of these membranes for adsorption of a metal ion such as nickel (II) assuming the size of 6 xc3x85 for the hydrated metal ion species and an internal surface of 100 m2/gm of membrane, yields a maximum surface entrapment capacity of 0.034 grams of nickel per gram of membrane. This, of course, is too low for efficient liquid volume reduction. In fact, where only single complexation sites are available, one will require a relatively high surface area of membrane (approximately 3000 m2/gm) in order to achieve a 1 gram of nickel uptake per gram of membrane.
In U.S. Pat. No. 4,604,204 to Linder et al., a cellulose acetate containing membrane having pore sizes of preferably 10-500 angstroms is treated with reagents such as di-aldehydes and diisocyanates that react with the hydroxyl groups of the membrane. The reagents function as linker molecules for the attachment of a polyfunctional oligomer or polymer. These membranes are made to exclude metal ions from pores rather than to entrap metals inside the pores.
While effective in excluding metal ions, this approach has several shortcomings. First, it should be appreciated that the linker molecules fill some space and tend to interfere with and close the relatively small diameter pores to subsequent reaction. Second, it should be appreciated that many times both functional groups of the linker molecules react with hydroxyl groups of the membrane leaving none available to subsequently react with the polyfunctional oligomer or polymer. Thus, the number of available sites for polyfunctional oligomer or polymer attachment is, in fact, quite limited thereby limiting the effectiveness of the modified membrane.
Still further, any cellulose based membrane suffers from an unacceptable degree of acid and/or solvent instability which limits or prevents its use in many applications. Thus, no form of chemical modification makes their use acceptable in these environs.
A need is therefore identified for an improved chemically activated microfiltration membrane that may be utilized for heavy metal ion sequestration and other purposes (e.g. nitrate ion sequestration) and that is characterized by a relatively high entrapment capacity heretofore unavailable in the art.
Accordingly, it is a primary object of the present invention to provide a chemically activated microfiltration membrane characterized by significantly enhanced surface entrapment capacity that is relatively easy to produce.
Another object of the present invention is to provide a chemically activated microfiltration membrane wherein polyamino acids are chemically attached to the membrane including within the pores in order to provide a relatively large number (e.g. 20-1000) of functional groups capable of ion entrapment per membrane attachment site.
Still another aspect of the present invention is to provide a unique and novel method for the preparation of high capacity chemically activated, microfiltration, composite polymer and silica-based membranes formed by means of the attachment of polyamino acids along the inside pore surfaces of the membranes.
Additional objects, advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention as described herein, an apparatus is provided for ion entrapment. The apparatus comprises a chemically activated microfiltration membrane constructed from a composite polymer and silica-based material. Such a silica-based membrane provides good stability in acids and solvents. Further, such a membrane is mechanically strong and resistant to both shrinking and swelling: problems which adversely effect organic resins such as sepharose and agarose.
The chemically activated microfiltration membrane includes a polyamino acid (e.g. polyglutamic acid, polyaspartic acid, polylysine, polyarginine, polycysteine and mixtures thereof) attached thereto. This attachment is by reaction of the terminal amine group of the polyamino acid with the membrane and, more specifically, an epoxide group on the membrane.
Specifically, the chemically activated, silica-based, microfiltration membrane is prepared by first removing any coating of oil on the membrane. This is followed by permeating the membrane with a solution of silane and a solvent so as to react methoxy groups of the silane with silanol groups of the membrane and thereby incorporate epoxide groups. Next is the removing of any residual silane. This is then followed by attaching a polyamino acid to the membrane by reacting a terminal amine group of the polyamino acid with an epoxide group on the membrane. This makes a strong, stable bond. Preferably, the membrane incorporates pores having a diameter of at least 1,000-6,000 angstroms so that the individual polyamino acid molecules may be attached to the membrane within the pores, even at pressures below 1 bar.
In accordance with still another aspect of the present invention, the method may include the step of regeneration of the membrane after metal entrapment by utilizing helix-coil properties of polyamino acids. This phenomena has been demonstrated with polyamino acids such as poly-L-aspartic acid and poly-L-glutamic acid.
The effects of helix formation allow one to close the interstitial spacing between ionized carboxylic acid groups of the polyaspartic acid and polyglutamic acid. Providing that the attached polyamino acids are not fully protonated, their electrostatic fields may be enhanced and thus, their overall effectiveness as a sorbent is likewise enhanced. Additionally, the helix formation also promotes radial chain expansion and the formation of a void near the center of the helix capable of sequestering cations because of the surrounding negative electrostatic field.
By utilizing this method it is possible to advantageously provide a membrane based sorbent wherein the available ion binding sites are multiplied and, therefore, the sequestration capacity of the membrane is significantly enhanced, perhaps between twenty and a hundred fold.
Still other objects of the present invention will become apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of this invention, simply by way of illustration of one of the modes best suited to carry out the invention. As it will be realized, the invention is capable of other different embodiments and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.